1. Field of the Invention
The present invention generally relates to an optical apparatus and a method for producing the same. Specifically, the present invention relates to a light generator including a semiconductor laser and a waveguide type optical function device, and a method for producing the same. The present invention also relates to an oscillation wavelength stabilizer for a light source such as a semiconductor laser having a distributed Bragg-reflector (DBR) region, a harmonic output stabilizer for a short wavelength light source which includes a semiconductor laser having a DBR region and a wavelength converting device, and further to an optical disk system including the same.
2. Description of the Related Art
In the optical information processing field, optical functional devices for modulating light output from a semiconductor laser at high speeds or halving the wavelength of laser light have been vigorously developed. In particular, waveguide type optical functional devices are promising for realizing the modulation of laser light at a frequency of several gigahertz or more or obtaining 1 mW or more of short-wavelength laser light. Hereinbelow, a waveguide type second harmonic generation (SHG) device (Mizuuchi et al., IEEE, Journal of Quantum Electronics, 30 (1994), pp. 1596) and an optical modulation device will be briefly described.
A typical SHG device 50 will be explained referring to FIG. 16, which is a perspective view of the SHG device 50.
The SHG device 50 includes a z-cut LiTaO.sub.3 crystal substrate 31. A waveguide 32 and periodical domain inverted regions 33 extending perpendicular to the waveguide 32 are formed on the z-cut LiTaO.sub.3 crystal substrate 31. The SHG device 50 allows harmonics to be generated with high efficiency by compensating the unmatching of the propagation constant between a fundamental wave and a generated harmonic with the periodical structure of the domain inverted regions 33.
Such an SHG device 50 is fabricated in the following manner.
A Ta electrode pattern is formed on the z-cut LiTaO.sub.3 crystal substrate 31 (made of nonlinear optical crystal) by evaporation and photolithography having a periodic spacing of several micrometers. A voltage of 2 kV/mm is then applied to the resultant substrate to form the periodic domain inverted regions 33. Thereafter, a stripe made of Ta, extending perpendicular to the periodic domain inverted regions 33, is formed on the substrate. The resultant substrate is heat-treated with pyrophosphoric acid for about 16 minutes, subjected to proton exchange, and annealed at about 420.degree. C. for about one minute, to form the waveguide 32.
The proton-exchanged waveguide 32 allows only light having a polarized component in the z direction to propagate therethrough. In general, the SHG device using the z-cut LiTaO.sub.3 crystal substrate has a higher conversion efficiency into a harmonic, though the SHG device can also be fabricated using an x-cut LiTaO.sub.3 crystal substrate.
A conventional light generator including such a waveguide type SHG device and a semiconductor laser will now be described.
Referring to FIG. 17, light emitted from a semiconductor laser 34 is guided into a waveguide 40 having a waveguide type SHG device 39 via two coupling lenses 35 and 38. More specifically, light emitted from a semiconductor laser 34 is collimated by a collimator lens 35, passes through a half-wave plate 36 and a bandpass filter 37, and is focused on the waveguide 40 of the waveguide type SHG device 39, by a focusing lens 38. The semiconductor laser 34 oscillates at a TE mode, while the waveguide 40 allows only a light component polarized in the z direction to propagate therethrough. The half-wave plate 36 is used to obtain the maximum overlap between the light emitted from the semiconductor laser 34 and the light propagating through the waveguide 40. The laser light emitted from the semiconductor laser 34 is converted into harmonic light while propagating in the waveguide 40 and output from the emergent end face thereof.
This conventional light generator generates about a 2.8 mW blue light with a wavelength of 430 nm from 120 mW laser light with a wavelength of 860 nm emitted from an AlGaAs semiconductor laser as the fundamental wave. The module volume of this light generator using the above two coupling lenses 35 and 38 is about 3 cc (Kitaoka et al, The Review of Laser Engineering, 23 (1995), pp. 787).
As optical disk systems and optical communication systems have been generally and widely used, demands for reducing the size and cost of relevant components have increased. In order to reduce the size and cost of the light generator including the semiconductor laser and the waveguide type optical functional device, it is required to simplify the optical coupling adjustment (i.e., the alignment adjustment for obtaining the optical coupling) and omit any coupling lens. In the conventional module structure, as shown in FIG. 17, using two coupling lenses 35 and 38 for guiding laser light of the semiconductor laser 34 to the waveguide 40, there are required four axial adjustments: i.e., adjustments of the focusing lens 38 and the collimator lens 35 both along the optical axis (direction y in FIG. 17); and adjustments of the semiconductor laser 34 in directions x and z in FIG. 17. Thus, a certain period of time is required for realizing the adjustment, and fabrication cost is increased. Further, the module structure including two coupling lenses has a relatively large volume of about 3 cc and occupies a relatively large space. The time and cost required for the optical coupling adjustment and the module volume of about 3 cc of the conventional optical functional device are disadvantageous in the application of the device to consumer appliances such as optical disk systems.
A direct-coupling module including no coupling lens has been proposed to achieve size reduction (Japanese Patent Publication No. 5-29892). This type of module, however, still requires alignment adjustments for optical coupling along two or three axes, requiring time and cost for the optical coupling.
Another problem of the conventional light generator is that presently it is difficult to optically couple the waveguide formed on the z-cut crystal substrate by proton exchange and the semiconductor laser light oscillating at the TE mode on one submount in a simple manner. The waveguide formed on the z-cut crystal substrate by proton exchange allows only light of a TM mode to propagate therethrough. Therefore, a half-wave plate is required to mount both the waveguide and the semiconductor laser emitting light of a TE mode on one submount.
By the way, optical disk systems using a near infrared semiconductor laser with a wavelength of a 780 nm band or a red semiconductor laser with a wavelength of 670 mm have been vigorously developed. In order to enhance the density of an optical disk, it is required to reproduce smaller spots. To reproduce smaller spots, a higher numerical aperture (NA) of a focusing lens and a shorter-wavelength of a light source are required. One of the conventional techniques for shortening the wavelength is second harmonic generation (SHG), where a near infrared semiconductor laser and a domain inverted type waveguide device of a quasi-phase matching (QPM) method is used (see Yamamoto et al., Optics Letters, Vol. 16, No. 15, 1156 (1991)).
FIG. 34 is a schematic structural view of a blue light source using a domain inverted type waveguide device (the SHG blue laser).
Referring to FIG. 34, the light source includes a 0.85 .mu.m-band, 100 mW-class AlGaAs DBR semiconductor laser 123, a collimator lens 124 with an NA of 0.5, a half-wave plate 125, a focusing lens 126 with an NA of 0.5, and a domain inverted type waveguide device 127. The DBR semiconductor laser 123 includes a DBR region for fixing the oscillation wavelength. The DBR region has an internal heater for varying the oscillation wavelength. The domain inverted type waveguide device 127 includes a waveguide 129 and periodic domain inverted regions 130 formed on an LiTaO.sub.3 substrate 128. The polarized direction of laser light collimated by the collimator lens 124 is rotated by the half-wave plate 125, and the resultant light is focused on the incident end face of the waveguide 129 of the domain inverted type waveguide device 127. The focused light propagates through the waveguide 129 having the domain inverted regions 130, to be output from the emergent end face of the waveguide 129 as a harmonic and a non-converted fundamental wave.
In the domain inverted type waveguide device 127, the phase-matching wavelength allowance where high efficiency wavelength conversion is possible is as narrow as 0.1 nm. To overcome this problem, the current amount supplied to the DBR region of the DBR semiconductor laser 123 is controlled, to fix the oscillation wavelength within the phase-matching wavelength allowance of the domain inverted type waveguide device 127. As a result, typically, about a 3 mW blue light with a wavelength of 425 nm is obtained for 70 mW laser light incident on the waveguide 129.
The DBR semiconductor laser includes an active region for obtaining a gain and the DBR region for controlling the oscillation wavelength. The DBR region is made of a material having a band gap larger than that of the active region so that the DBR region is transparent for the laser light with a wavelength of 850 nm. Since the DBR region has a lattice, the oscillation wavelength is controlled by the wavelength of the light reflected from the DBR region.
Thus, the oscillation wavelength can be varied by varying the refractive index of the DBR region. The refractive index of the DBR region can be varied by methods such as (1) supplying a current to the DBR region, and (2) varying the temperature of the semiconductor laser using an electronic cooling element (Peltier element) and the like.
However, in the case of varying the oscillation wavelength by varying the DBR current, it may difficult to precisely fix the oscillation wavelength at a desired value.
On the other hand, in the case of varying the oscillation wavelength by varying the temperature of the semiconductor laser, an electronic cooling element having a heat absorption capacitance of about 1 W is required for the control of the temperature of the semiconductor laser, which increases power consumption. Moreover, when the ambient temperature under which the semiconductor laser is used becomes wider, operational reliability may be deteriorated. In addition, output intensity of laser light varies as the temperature of the semiconductor laser varies for changing the oscillation wavelength. If the current supplied to the active region is adjusted to compensate the output variation, the phase condition changes, causing mode hopping.
In order to avoid the mode hopping, a semiconductor laser having a phase control section has been proposed. However, it is likely to be difficult to find a control method for stably and continuously varying the oscillation wavelength irrespective of a variation of the ambient temperature.
In the short-wavelength light source including the semiconductor laser having the DBR region and the wavelength converting device, it is required to match the oscillation wavelength of the semiconductor laser with the phase-matching wavelength of the wavelength converting device. In the short-wavelength light source, the output of short-wavelength light varies if the oscillation wavelength shifts from the phase-matching wavelength. In particular, when a quasi-phase matching (QPM) type wavelength converting device having a periodic domain inverted structure is used as the wavelength converting device, it is significantly important to control the oscillation wavelength of the semiconductor laser because the allowance for the phase-matching wavelength is as narrow as about 0.1 nm.
Another problem of the short-wavelength light source is that LiTaO.sub.3 crystal and LiNbO.sub.3 crystal used for the substrate of the QPM type device have optical damage characteristics. The optical damage as used herein refers to the variation of the refractive index caused by irradiation with short-wavelength light. As the refractive index varies, the phase-matching wavelength of the QPM type device shifts. In order to obtain a stable harmonic output, therefore, it is required to always control the wavelength of the semiconductor laser to match with the phase-matching wavelength.